All roadway bridges whether new or existing must be load rated to determine safe live load carrying capacity. A load rating analysis may be performed as part of the design process for new structures. Load rating for existing structures is reviewed regularly and updated based on the observed condition of the superstructure. Detailed calculation procedures, as published by AASHTO and/or the various state highway departments, are followed. In critical cases which warrant the expense, field verification testing may be performed. These procedures do not address substructure load rating other than to point out that the engineer may want to consider it if there is evidence of deterioration. The methodology to be used in this situation is undefined and left to a bridge engineer's judgment. As a result, the substructure load rating is often based on a qualitative rather that a quantitative approach and may not account for soil behavior and soil-structure interaction. There is currently no procedure specific to substructure load rating. Most bridge load ratings are based on analysis of the superstructure only.